Introduction

In recent years, the intense focus has been given to finding sustainable energy solutions that are cost-effective and have a minimal environmental impact, and, and are abundant in nature. Several energy harvesting techniques, such as piezoelectricity,[1]triboelectricity,[2]thermoelectricity,[3] have been thoroughly explored to convert mechanical, thermal, and renewable energy into electricity. However, each approach has barriers, such as extensive material costs, complicated preparation procedures, limited resources, and inadequate electrical output, which limit their practical use. It is crucial to maximize the energy extraction from a pure source for long-term feasibility. Because of its abundance (71% of the Earth’s surface) and significant intrinsic renewable energy potential, water has gained increasing interest for its use in prominent power generation. While traditional approaches like thermal energy extraction,[4] or hydroelectricity,[5,6] generation has been widely exploited, hydrovoltaic method has recently emerged as a viable method for harnessing energy.[7-10] This unconventional hydrovoltaic system can harness electricity through the direct interaction between functional materials and various forms of water, such as water droplets,[11] flow of water,[12,13] wave,[14,15]evaporation,[16-19] and moistures.[20] Evaporation-based energy harvesting from water droplets has recently gained attention in relation to the hydro-voltaic effect.[7,8]Utilizing the interactions between the water droplet and solid material interfaces, this method can generate electricity by leveraging physical phenomena such as streaming potential,[21] ionic motion coupling,[22]triboelectrification.[23] Since efficient energy harvesting from sustainable sources is crucial, evaporation-driven electrical generators (EEGs) are a potential solution for the progress of renewable energy systems. This EEG process is more spontaneous and can utilize maximum energy from water.[17] Several novel nanomaterials have garnered significant attention in the realm of hydrovoltaic energy harvesting, which yields electrical potential at the interface between water and a polarizable substance.[24] Despite the promising characteristics of the novel nanomaterials, such as high conductivity and surface area, their feasibility is limited by their high costs, limited availability, resistance to water flow, and labor-intensive preparation processes. Over the years, naturally available bio-based materials having porous structures were overlooked. Recently, microchannels of different natural wood structures have been exploited for hydrovoltaic energy harvesting.[25-27]However, most of the existing organic material-based WEGs exhibit lower power density, hindering their suitability for practical device application. Moreover, the the device size would be substantially larger because of the required shape of the wood[27] and the required amount of water is considerably higher. On top of that, some forms of pretreatments are required to improve the power density.[26]In this study, we investigate a new approach- evaporation-driven Water induced Electric generators (WEG) by utilizing natural nutshells, which possess a porous micro/nanochannel structure[28]and sufficient polar functionalities.[29] This natural anisotropic 3D micro/nano-channel architecture of nutshells (NSs) facilitates efficient water and nutrient transport.[30] For the first time, the micro/nanochannels within the NSs (Almond Shell - AS, Filbert Shell - FS, Pecan Shell - PS, Walnut Shell - WS) have been successfully utilized to harness hydrovoltaic energy through evaporation by utilizing the concepts of streaming potential as illustrated in Figure 1 . A simple one-step preparation process can harvest above 600 mV with a power density of 5.95 µW·cm−2. The efficiency of this device is affected by critical factors such as density, porosity, contact area, surface charge, and hydrophilicity. By applying some chemical treatments, the surface area and porosity of the nutshells (NSs) have been augmented. The subsequent highly porous shells exhibit an impressive output potential of 1.21 V and a maximum current density of 347.2 µA·cm−2 under the synergistic effect of streaming potential and chemical reactions. These innovative and functional NS-powered energy generators are cost efficient and environmentally friendly, rendering them suitable for powering small electronic devices. Moreover, this innovative technology presents a novel approach to green energy solutions and broadens the range of materials that can be potentially utilized for Water induced Electric Generators (WEGs).